Scientists in the UK have dated a set of footprints found in
1961 in the Chihuahuan desert in northeastern Mexico helping us understand the
climate conditions in this area more than 7,000 years ago. The footprints were
excavated while workman were building a road and placed in the Museo del
Desierto in Saltillo, Coahuila. The age of the footprints piqued the interest
of researchers at the John Moores University in Liverpool. In 2006 their
curiosity yielded a second set of prints in a Cuatro Ciénegas quarry.

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'When we discovered these new prints, they were preserved in
the same material as the ones in the museum. So we presumed it was a
rediscovery of these lost footprints as opposed to a new discovery,' explains
Dr. Nick Felstead now of Durham University, lead researcher on the project. But
they needed to prove it. So, using oxygen and carbon isotopes in the
surrounding material they worked out the age of the prints.

'The age of the prints in the museum had been given a
best-guess at being around 10 to 15,000 years old, but they had never actually
been dated,' says Felstead. 'The two sets of dates came back at 10.5 thousand
years and seven thousand years old, so by age alone we knew they were separate;
they couldn't have been same trackways.'

With the two sets of prints now dated, the researchers set
about determining the climate during the times these prints were made.

They first determined that the earlier set of prints was preserved
in carbonate-rich sediments from a marshy region. The specific sediments were
identified as mostly travertine. Travertine precipitates out when water
percolates through limestone suggesting that the area at that time was much
wetter than it is today.

Further the water, which formed the travertine, also
contains minute traces of uranium. Over
time uranium decays and turns into thorium. The scientists measure the ratio of
uranium to thorium to determine how old the age of the footprints.

'It's in the middle of the Chihuahua desert, everyone always
thinks that deserts are hot, arid and hostile but these footprints show us that
during the Holocene, the desert was just coming out of a period of glaciation
and had only just started to dry out,' Felstead says. 'It's a window into a
time when the desert was wet enough to support a much greater range of life.'